Fatigue in Plastic Field of AISI 304L on an Engine Component: An Interesting Case on a Pulley for Automotive Application

This study investigates the failure mechanisms of a press-fitted AISI 304L pulley, which is used to drive an engine coolant variable water pump in automotive applications. The analysis focuses on the peculiar loading scheme of the pulley resulting from the innovative water pump design which combines high mean stress from press fitting with cyclic stress from rotating bending loads that exceed the material's yield point. This is coupled with the cyclic material behavior of AISI 304L which exhibit a strong cyclic hardening. This combination significantly influences the stress distribution and fatigue life of the component under cyclic loads combined with material plasticity, ultimately leading to fatigue failure at the pulley-shaft mating surface. Assembly endurance tests were conducted on a specialized test bench, allowing control of pulley bending load. A comprehensive failure analysis, including visual inspection, metallurgical examination, and finite element analysis (FEA) was performed to determine the root cause of the failure. Visual inspection revealed material loss, crack sites and metal debris at the mating surface, attributed to cyclic loading and yielding. FEA S-N fatigue model results demonstrated high-stress concentrations at the mating surface, with cyclic stress superimposed on high mean stress, leading to overcome the yielding point of the material. The calculated critical stress location is aligned with observed damage sites and there is a quantitative agreement between estimated fatigue life and test bench endurance test results. The conclusions drawn from this failure analysis provide valuable insights for the design and material selection of press-fitted pulleys in similar applications. Recommendations for improving the fatigue resistance include optimizing load distribution through design modifications and selecting materials with more favorable cyclic characteristic.